The present invention relates to digital oscilloscopes and in particular to a digital oscilloscope which automatically adjusts a waveform display so that it is adequately representative of an input signal.
In a typical digital oscilloscope, a vertical amplifier amplifies an input signal to produce an output signal, and this output signal is continuously sampled and digitized by a digitizer. Digitized sample data is stored at successive addresses in an acquisition memory. When the current address reaches an upper limit, it is reset to a lower limit and newly acquired sample data is written over previously stored data. A waveform data sequence stored at a particular section of acquisition memory addresses is periodically read out of the acquisition memory and utilized to control the display of a waveform, the magnitude of successive data of the sequence controlling the vertical position on the screen of successive segments of the waveform. The section of acquisition memory addresses providing the waveform data sequence controlling the display is selected to include a set number of sample data values acquired starting with an adjustable number of samples after the amplifier output signal reaches an adjustable trigger level. This permits adjustment of the "horizontal position" of the waveform display on the oscilloscope screen with respect to the triggering event on the waveform.
A waveform display is often considered to be most representative of a periodic input signal when its minimum and maximum peaks appear on the screen and when the vertical distance between minimum and maximum waveform peaks is as large as possible. Also it is desirable that the horizontal time scaling be such that at least one complete cycle of the waveform appears on the screen. However, display of too many cycles can obscure waveform detail due to limitations of display resolution. The trigger level should be set between the maximum and minimum peaks of the amplifier output signal and at a level wherein noise is not likely to cause premature triggering. It is also beneficial to adjust the horizontal position of the waveform so that the triggering event appears other than at one end of the waveform display so that it may be easily seen.
Typically an operator adjusts the horizontal time scale of a waveform by changing the sampling rate at which an input signal is digitized, adjusts the vertical scaling of the waveform display by changing the gain of the vertical amplifier, and adjusts the vertical position of the waveform by changing the magnitude of an offset signal input to the vertical amplifier which causes the amplifier output signal to be shifted upward or downward in magnitude. The operator can also adjust the trigger level by changing the magnitude of a trigger level signal which is compared with a trigger signal (usually the vertical amplifier output signal) to produce a trigger status signal utilized to control termination of waveform digitization. The operator can usually adjust the horizontal position by providing position control data to digitizers.
It is not always easy to properly adjust all of these waveform display control parameters so as to obtain a suitably representative waveform display, nor is it easy to tell when a display is suitably representative, particularly when nothing is known beforehand about the characteristics of an input signal. When the input signal is initially amplified and digitized, it may be that no waveform actually appears on the oscilloscope screen, as for example when the vertical offset signal is too large or too small. Or, if the sampling rate is too high, only a portion of one cycle of the amplifier output signal may be digitized and a periodic input signal may initially appear to be constant. An input signal may also appear to be constant if the gain is set too low. If an input signal is periodically digitized with the trigger level improperly adjusted, the resulting waveform display may be unsteady.
An experienced oscilloscope operator will typically adjust the sampling rate, vertical amplifier gain and offset, trigger level and horizontal position in a systematic fashion to ensure that a suitably representative waveform display is obtained. But an inexperienced oscilloscope operator may not understand all of the adjustments which must be made and may not be aware that a waveform display is not truly representative of the input signal.
When an oscilloscope can display several waveforms, it may be impractical to provide a separate front panel knob or switch for separately controlling each parameter controlling each waveform as well as for controlling other aspects of oscilloscope operation. Some oscilloscopes provide multiple function knobs or switches and/or menus operating in "decision tree" form to permit an operator to control oscilloscope operation utilizing only a few knobs or pushbuttons. In order to change a display attribute of a particular waveform, an operator must first utilize a knob, pushbutton or menu to select the waveform to be adjusted, utilize another knob, pushbutton or menu to select the particular aspect of the waveform display to be changed, and then utilize yet another knob, pushbutton or menu to actually adjust such aspect of the waveform. While oscilloscopes utilizing decision tree control systems minimize the number of front panel pushbuttons and knobs needed to adjust a waveform display, the waveform display adjustment process in such oscilloscopes may be slow and cumbersome even for an experienced oscilloscope operator.
What is needed is an oscilloscope which can automatically adjust a waveform display so as to ensure that it is suitably representative of an input signal.